This invention relates to a broadband network system wherein a single transmission path is used commonly for a plurality of channels and one or more channels are used for effecting transmission of signals of the baseband network.
In recent years, a broadband system in which a plurality of communication units are connected in parallel to a common transmission path has become noted in the art. Among references regarding this technique may be mentioned a paper of the title "Broadband coaxial local area networks", Computer Design, July, 1980 and a paper of the title "Broadband technology magnifies local networking capability", Data Communication, Feb. 1980.
Among the broadband systems are included a single line system wherein a single coaxial cable is used in common for a transmission line and a receiving line by employing different channel frequencies, and a two-line system wherein signals are transmitted through two coaxial cables by employing the same channel frequency for the transmission line and the receiving line.
As shown in FIG. 1, according to the single line system, a plurality of communication units 2 are connected in parallel with a single transmission path 1 and an all-channel central retransmission facility (hereinafter referred to as CRF) 3 for sending back all channels is connected to one end of the transmission path 1. When a high frequency transceiver contained in each communication unit sends out a signal having a frequency f1, which is a representative of various assigned frequencies, the transmitted signal is converted to a receiving signal having a frequency of f2 by the CRF 3 and the receiving signal is returned to be received by a radio frequency modem (RF modem) of another communication unit to perform communications.
According to the two-line system, as shown in FIG. 2, a plurality of communication units 12 are connected in parallel with two transmission paths 11 including a return channel transmission path 11a and a forward channel transmission path 11c, and an all-channel CRF 13 for sending back all channels is connected to one end of the transmission paths 11. When an RF modem in each communication unit 12 transmits a signal of a (representative) frequency of f11 to the return channel transmission path 11a, the signal of the frequency f11 is amplified by the CRF 13 and then returned to the forward path 11C to be received by an RF modem of another communication unit thereby performing communications.
An existing broadband transmission path comprises a plurality of frequency divided channels and has a length of several kilometers to several of tens of kilometers.
In such a long transmission path, the probability of mutual communications between remotely separated communication units is lower than that of closely adjacent communication units so that there is a tendency of constituting communication networks i.e., areas denoted at reference characters A, B and C in FIG. 1 in which mutual communications are made between relatively close communication units. Where the transmission path becomes so long that the amount of attenuation of signals increases, repeaters are installed, in some applications, at intermediate points of the transmission path.
Known baseband transmission systems include carrier sense multiple-access with collision detection (CSMA/CD) system and a carrier sense multiple access (CSMA) system. Since each of these systems utilizes a probability method of acquiring priority of transmission by avoiding collision of a plurality of signals, the time constraints on parameters regarding transmission is severe so that it is necessary to limit the length of the transmissions path to a value of only several kilo meters which is smaller than that of the broadband transmission path.
In this manner, the broadband network system can employ a single transmission path over a relatively long distance. But as the distance between the communication unit 2 or 12 and the all-channel CRF 3 or 13 increases, the propagation delay time increases correspondingly so that when communications are made in a CSMA/CD system, typically examplified by Ethernet wherein constraints on the propagation time delay exist, there is a defect in that the communications of this type can be made only between the communication units near the CRF.
Moreover, there is a defect in that in the baseband network on a broadband network system, communications over a relatively long distance can not be made because of constraints on the propagation delay time.